Android AsyncTask vs Thread + Handler vs rxjava

I know this is the question which was asked many many times. However there is something I never found an answer for. So hopefully someone can shed me some light.

We all know that AsyncTask and Thread are options for executing background tasks to avoid ANR issue. It is recommended that asynctask should only be used for short-running tasks while thread can be used for long-running tasks. The reasons why asynctask shouldn't be used for long tasks are well-known which is about the possible leak caused by asynctask since it may continue running after an activity's destroyed. That is convincing. However, it also leads to some other questions:

Isn't thread also independent from activity lifecycle? Thus, the risk with asynctask can also be applied to thread. So why thread is suitable for long-running tasks?

Looks like the risk of asynctask is only applicable when using it with activity. If we use it in service (not IntentService since IntentService stops after its work's completed), and as long as we can guarantee to cancel the asyntask when the service's stopped, can we use it for long-running tasks? and doesn't it means it's risk free to use asynctask in services?

I've played with rxjava for a while and really like it. It eliminates the need of worrying about threading (except you have to decide in which thread to subscribe and observe the emitted data). From what I can see, rxjava (in conjunction with some other libs like retrofits) seems to be a perfect replacement of asynctask and thread. I'm wondering if we could completely forget about them or there is any specific case that rxjava can't achieve what asynctask and thread can do that I should be aware of?

The reason why AsyncTask is recommended for only short tasks (around 5 seconds) is there is no method to cancel a running AsyncTask. There exists a method called AsyncTask.cancel(true) which invokes onCancelled(Result result). However, according to the docs, this method "runs on the UI thread after cancel(boolean) is invoked and doInBackground(Object[]) has finished." (https://developer.android.com/reference/android/os/AsyncTask.html). On the other hand, Thread can be stopped with Thread.interrupt().

There shouldn't be any problem running an AsyncTask within a Service provided that you are aware of the cancellation limitation of AsyncTask and the possibility of memory leak can be created by AsyncTask. Note that, there is obviously no need to use an AsyncTask in an IntentService which is already running in a worker thread.

This is a very experience-based question. I guess there would be no complete answer. What we can do is to understand Rx and being aware of the its limitations to determine where suitable to use it. In my development work, I use RxJava all the time without having any issue. Note that the same memory leaking issue is also applied to RxJava. You can perhaps find one of the specific questions here. There are also a whole bunch of discussions about handling leaking/screen rotation with RxJava that can be easily found by Googling.